1. Field of the Invention
The present invention relates to a method of bonding a fluoro-containing compound to a silicone rubber substrate with a primer composition comprised of an organic based titanate compound and an alkoxysilane compound, as well as to a bonded composite of the foregoing materials.
2. Discussion of Related Art
In the art of electrophotography/electrostatography, heat-softenable toners are widely used in imaging methods wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Following development of the latent image with the toner, the toner is transferred in image-configuration to a surface of an image receiving medium such as, for example, paper or transparent film. The toner image is then fixed in place upon the medium to yield the final image.
A typical method of fixing toner in place on the medium involves applying heat to the toner once it is on the image receiving medium surface to soften it and then allowing or causing the toner to cool. One such fusing method comprises passing the toner-bearing medium sheet through the nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser member or roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure member or roll) serves to press the receiver sheet into contact with the fuser roll. The fuser member and pressure member may also take the form of a flat plate or belt.
A fuser member can comprise, for example, a rigid core covered with a resilient material, particularly a silicone rubber. In addition, in electrophotographic fuser systems, fuser members are commonly made with an overcoat layer of a polyfluorocarbon resin or polyfluorocarbon elastomer. Fluorocarbon resins like polytetrafluoroethylene (PTFE) or a copolymer of PTFE and perfluoroalkylvinylether, or fluorinated ethylenpropylene have excellent release characteristics due to very low surface energies, high temperature resistance, and excellent chemical resistance. Polyfluorocarbon elastomers, such as vinylene fluoride-hexafluoropropylene copolymers, are tough, flexible elastomers that have excellent high temperature resistance.
However, due to the low surface energies of the silicone rubber and the fluoro-containing material overcoat, it is very difficult to obtain a good bond between these layers. This is a particular problem in the area of fuser members for electrophotographic and electrostatographic devices where strong adhesion between a silicone rubber and a fluoroelastomer topcoat is necessary for long-term, reliable operation.
U.S. Pat. No. 5,595,823 describes a fuser member useful for heat-fixing an electrographic toner to a substrate. The fuser member has a core and an outer layer. The outer layer includes a cured fluorocarbon random copolymer and particulate filler of aluminum oxide and a material selected from the group consisting of alkali metal oxides, alkali metal hydroxides, and combinations thereof. In a specific embodiment, a silicone rubber layer is coated upon a stainless steel substrate core, the core being primed with a uniform coat of a metal alkoxide type primer, Dow 1200 Prime Coat primer, marketed by Dow Corning Corporation of Midland, Mich., which is described to contain light aliphatic petroleum naptha (85 weight percent), tetra (2-methoxyethoxy)-silane (5 weight percent), tetrapropyl orthosilicate (5 weight percent), and tetrabutyl titanate (5 weight percent).
U.S. Pat. No. 5,599,631 describes a fuser member having as its outermost layer a substantially homogeneous composite comprising: a) a continuous phase of a fluorocarbon elastomer selected from vinylidene fluoride based fluoroelastomers containing hexafluoropropylene and terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene; and b) a discontinuous phase of a fluorinated resin having a molecular weight between about 4,000 and 25,000. Again, it is indicated that a silicone rubber intermediate layer may be applied to a core primed with Dow 1200 primer.
U.S. Pat. No. 5,973,067 describes a primer composition for promoting the adhesion of thermosetting rubbers to substrates having exceptionally inert surfaces, such a nickel, stainless steel and organic resins. The composition comprises: (A) 100 parts, by weight, of an organosilicon compound selected from the group consisting of (1) alkenyltrialkoxysilane and (2) mild hydrolyzates of (1); (B) 0.01-200 parts, by weight, of a platinum compound; (C) 0.1-100 parts by weight of an organic titanic acid ester; and optionally (D) a solvent.
The foregoing patents thus describe primer compositions intended to assist in bonding a silicone rubber to a metal surface.
U.S. Pat. No. 5,238,708 describes a primer composition, and a method for its use to improve adhesion between a silicone substrate and a subsequently applied paint film. The primer composition consists essentially of a mixture of an alkoxysilane, an organotitanate and a tin salt of a carboxylic acid, each component being present in a specific critical molar proportion. Although this patent mentions the use of the primer to enhance adhesion between a silicone rubber and a paint overcoat, it does not indicate that the paint should contain a fluoro-containing compound. Further, it is not taught that the solvent of the primer is a solvent that swells the silicone rubber.
U.S. Pat. No. 3,671,483 discovered that a solution containing a tetraalkyltitanate and a particular silane which has at least one group of the formula CH2CH2xe2x80x94Si(OOCCH3)3 in its molecule is useful as a primer in adhering a silicone rubber to epoxy and polyurethane.
U.S. Pat. No. 4,681,636 discloses a primer composition particularly suitable for bonding fluorosilicone rubber to a substrate. This primer comprises a solution of a t-butyl peroxy-functional silane, a reactive organosilicon compound having at least two alkoxy groups and an organotitanate ester. U.S. Pat. No. 4,749,741 describes primers suitable for promoting adhesion between silicone rubber and plastics or metals. These primers comprise a solution of an unsaturated organotitanate ester and either an organosilicon compound having at least two alkoxy groups or an organohydrogensiloxane (or silane).
Until now, primers used in bonding a overcoat to a silicone rubber still suffer from adhesion failures between the layers during operation as fuser members. What is still desired, then, is development of a primer that can achieve enhanced bonding strength between a fluoro-containing compound overcoat and a silicone rubber substrate.
It is therefore an object of the present invention to develop a composite of a silicone rubber and a fluoro-containing material in which the bond between the layers is very strong. It is a still further object of the present invention to develop a fuser member containing such composite member for a copying device.
It is a further object of the present invention to develop a primer composition to achieve the desired enhanced bond strength between a silicone rubber and a fluoro-containing material.
These and other objects of the present invention are achieved herein by a composite structure comprising a layer of silicone rubber that is bonded to an overlying layer containing at least one fluoro-containing compound, the bond effected via a primer composition on a surface of the layer of silicone rubber, wherein the primer composition is comprised of at least an organic titanate and an alkoxysilane.
In a further embodiment of the invention, the invention pertains to a fuser member comprising a core overcoated with a first layer of silicone rubber that is bonded to an overlying layer containing at least one fluoro-containing compound, the bond effected via a primer composition on a surface of the layer of silicone rubber, wherein the primer composition is comprised of at least an organic titanate and an alkoxysilane.
In a further embodiment of the invention, the invention pertains to a method of obtaining a bond between a layer of silicone rubber and an overlying layer containing at least one fluoro-containing compound, the method comprising (1) applying a primer composition comprised of at least an organic titanate and an alkoxysilane to a portion of the layer of silicone rubber to which the layer containing at least one fluoro-containing compound will be applied, (2) fixing the primer composition to the layer of silicone rubber to obtain a primed portion, (3) applying the layer containing at least one fluoro-containing compound to the primed portion to form a composite structure, and (4) heating the composite structure.
Primers containing only a silane compound have proven insufficient in terms of bond strength in bonding a silicone rubber and a fluoro-containing compound. This is particularly problematic when the bond is required for materials to be used in a fuser member. Mechanical strain to facilitate paper stripping from the fuser member surface and the high temperatures required for toner fix when the paper is upon the fuser member are severe conditions demanding a high strength bond between materials of the fuser member in order to avoid failure of the fuser member after only a short period of operation of the copying device.
The present inventors have found that a primer composition comprised at least of an organic titanate and an alkoxysilane can prime the surface of the silane rubber and bond thereto, and can further form a strong bond to an overcoat containing a fluoro-containing compound.
The primer to be used in the present invention thus comprises at least an organic titanate and an alkoxysilane.
As for the organic titanate, it preferably has a formula of
Ti(OR)4
where R is an alkyl radical having from 1 to 8 carbon atoms. Specific examples of suitable organic titanates include tetrabutyl titanate, tetraisopropyl titanate, tetramethyl titanate, ethylmethyldibutyl titanate, tetraoctyl titanate and tetraethylhexyl titanate. In preparing the primer composition, it is most preferred that the organic titanate is tetrabutyl titanate.
The activity of the organic titanate in the primer is due to the dual functionality of both the titanate and the organic (e.g., butyl) structures of the compound. Both structures are hydrophobic and render the composition soluble in an organic solvent system, which insures a uniform composition. Further, the organic titanate serves to displace any water from the surface of the silicone rubber to which it is applied, which displacement serves to enhance the adhesion among the components.
As for the alkoxysilane, any known alkoxysilane compound may be used. Allyl alkoxysilanes are preferred, with allyltrimethoxysilane being most preferred. Examples of other alkoxysilanes that might be used include, for example, methyltrimethoxysilane, vinyltrimethoxysilane, butyltrimethoxysilane, hexyltrimethoxysilane, vinyltriethoxysilane, allyltriethoxysilane, methyltriethoxysilane, dimethyldimethoxysilane, methylvinyldimethoxysilane and methylallyldimethoxysilane.
The alkoxysilane compound also has dual functionality. The alkoxysilane function reacts with silicon atoms of the silicone rubber to which it is applied, forming a bond. The vinyl group (double bond) of the alkoxysilane reacts with fluorine atoms of the fluoro-containing compound that is applied upon the primed portions of the silicone rubber. Thus, a chemical bond is formed to link the dissimilar fluoro-containing compound to the silicone rubber material.
Although the primer composition can be applied (i.e., coated) directly to a silicone rubber at 100% solids, it is preferred to dilute the composition with one or more solvents in order to reduce the application viscosity and limit the thickness of the primer coat. Examples of suitable solvents include, for example, ketones, hexane, heptane, toluene, xylene and mineral spirits.
In a preferred embodiment, the primer composition contains a theta solvent that is capable of swelling the silicone rubber layer to which it is applied. A theta solvent is one where the free energies of the solvent-solvent, solvent-polymer and polymer-polymer interactions are the same. The solvent essentially swells the silicone rubber under layer beneath the surface of the silicone rubber, enabling the active components of the primer composition to penetrate beneath the surface of the silicone rubber. As a result, the bond will not just be at the surface of the silicone rubber, but will also be some distance beneath the surface of the silicone rubber. This permits a much stronger bond to be achieved compared to the use of a primer that binds only at the surface of the silicone rubber. Most preferably, a suitable swelling solvent is a ketone, in particular methyl isobutyl ketone.
In this preferred embodiment, the primer should be applied and permitted to sit for a short time (i.e., have a dwell time) to permit the swelling and penetration to occur prior to fixing the primer to the silicone rubber. It is preferred that the primer dwell time is at least, for example, about 15 minutes, and preferably no longer than about 8 hours.
The primer composition preferably includes from about 1 to 5 mole percent of an organic titanate and from about 3 to about 7 mole percent of the functional alkoxysilane. These components are thoroughly blended to form a homogeneous dispersion or solution. When a solvent or solvent mixture is employed, it is preferred that from about 0.5 to about 12 weight percent of the primer composition is thoroughly dispersed therein.
The primer may also contain other components as needed or desired such as, for example, dyes, pigments, flow additive, and the like.
To form the bond between the silicone rubber layer and the overlying fluoro-containing compound layer, the primer composition is first applied to a silicone rubber substrate layer by any suitable technique known to the art such as, for example, dip coating, spray painting, knife coating, brush coating, etc. The primer is most preferably applied only to those portions of the silicone rubber where a fluoro-containing compound will be bonded.
As the silicone rubber, any well known type of silicone rubber may be used, many of which are commercially available from numerous sources. The preferred silicone rubbers are known as addition cured such as LSRs or high temperature vulcanized known as HTV""s which have desirable thermal and mechanical properties.
In a preferred embodiment of the present invention, the silicone rubber is part of a fuser member of a copying (electrostatographic or electrophotographic) device. A preferred structure of a fuser member includes an aluminum substrate, for example an aluminum cylinder core having a diameter of, for example, about 100 mm. The silicone rubber is applied upon the aluminum core via any suitable method and, if desired, using any suitable type of primer. In a typical fuser member, the silicone rubber layer may have a thickness of from, for example, about 5 mm.
Following application of the primer composition to the silicone rubber, the primer is fixed to the silicone rubber through controlled heating and relative humidity conditions. Such control is needed in order to effect the hydrolysis reaction in which the silane is bonded with silicon atoms of the silicone rubber. Most preferably, the fixing occurs at a temperature of from about 65xc2x0 F. to about 75xc2x0 F. and at a relative humidity of from about 55% to about 65%. The conditions for dwell, as mentioned above, should preferably be at least about 15 minutes up to and including about 8 hours for essentially all solvent of the primer to be evaporated and for the adhesion reaction to reach an optimum.
Subsequent to the fixing, the fluoro-containing compound material may then be applied to at least the primed portions of the silicone rubber layer. Here again, any suitable application technique may be used such as those mentioned above.
By xe2x80x9cfluoro-containing compoundxe2x80x9d is meant any suitable compound that contains fluoro-substituted groups that can bond with the vinyl groups of the primer composition. Fuser members are often made with an overcoat layer of a polyfluorocarbon resin or polyfluorocarbon elastomer. Fluorocarbon resins such as polytetrafluoroethylene (PTFE) or a copolymer of PTFE and perfluoroalkylvinylether, or fluorinated ethylenepropylene, which have excellent release characteristics due to very low surface energies, high temperature resistance, and excellent chemical resistance, and polyfluorocarbon elastomers, such as vinylene fluoride-hexafluoropropylene copolymers, which are tough, flexible elastomers that have excellent high temperature resistance, may be used. In a preferred embodiment, the fluoro-containing compound is a fluoroelastomer. Commercially available fluoroelastomers include, for example, Viton fluoroelastomers from DuPont-Dow and Fluorel fluoroelastomers from 3M.
In a fuser member, the fluoro-containing compound layer is typically very thinly applied, for example to have a thickness of about 20 microns or so.
After the fluoro-containing compound layer is applied to form the composite structure, the composite structure is subjected to further heating in order to not only cure the fluoro-containing compound, but also to form the bond between the primer active components and the fluoro-groups of the overcoat. The heating cycle is, most preferably for example, 2 hours at 130xc2x0 F., 2 hours at 200xc2x0 F., 2 hours at 300xc2x0 F., 2 hours at 350xc2x0 F., 2 hours at 400xc2x0 F. and 6 to 12 hours at 450xc2x0 F.
By this method, a very strong bond can be formed between the two dissimilar materials of the silicone rubber and the fluoro-containing compound.